>> couric: tonight president obama warns the u.s. and nato are considering a military response to the crisis in libya as qaddafi's forces step up attacks on rebel-held oil towns. i'm katie couric. the violence is driving up the price you pay at the pump. but where's the money going? maybe not where you think, and should the u.s. tap the strategic reserves to bring prices down? they're here, the new 3-d mammograms. but should women be rushing to get one? and she's worn his m.i.a. bracelet for nearly 40 years waiting for the day she could remove it. that day has come. captioning sponsored by cbs from cbs news world headquarters in new york, this is the "cbs evening news" with katie couric. >> couric: good evening, everyone. libyan government forces were back on the offensive today trying to retake more territory still held by the rebels. some of the heaviest fighting was in towns near tripoli and the u.n. says more than a million libyans now need humanitarian aid. president obama warned moammar qaddafi and his supporters they will be held accountable for the violence and the u.s.